The present invention relates to thread forming screws in which threads are formed in a workpiece by plastic deformation. Such screws have applicability in a variety of workpiece materials, but are particularly adapted for use in soft metals and hard plastics. As with many screws, it is desirable that thread forming screws have relatively low driving torque and high stripping torque, because many of these kinds of screws are driven with settable automatically clutched drivers. The operating range of a screw type is defined as the difference between the maximum drive torque and the minimum strip torque. An increased operating range means that a clutch setting can be selected which will result in fewer screws that cannot be driven and fewer screws that will strip. Given the desirability of a large operating range, efforts in the screw art have been directed to decreasing drive torque while at the same time increasing the torque required to strip a screw.
Examples of thread forming screws include tri-roundular screws, as shown in U.S. Pat. Nos. 3,180,126 and 3,195,156. Other examples are shown in U.S. Pat. Nos. 3,218,905 and 3,878,759 which are directed to screws made from blanks having slightly deformed cross-sectionally symetrical tips.
It is an object of the present invention to provide a screw which creates a thread in a workpiece by plastic deformation.
Another object is to provide a thread forming screw which is easily manufactured.
Yet another object is to provide a thread forming screw which has a large operating range.
Still another object is to provide a method of making screws having the above characteristics.
The screw of the present invention is made from a screw having a pinched tip. The cross-sectional shape of the tip is assymetrical in order to create a particularly shaped leading edge which plastically deforms workpiece material without removing material on its way therethrough. By shaping the tip of a screw in accordance with the present invention, frictional contact between the screw and the workpiece is reduced, interference between the leading edge of the threads on the screw and the workpiece is minimized, and portions of the tip contribute to the holding power of the screw.